Officer who posed as green activist has lost everything

Undercover policeman who infiltrated campaign group says he now has no job, family or partner

The story of an undercover policeman leading a double life – cheating on his wife and "going native" – was never going to have a happy ending. Mark Kennedy, the policeman who posed for years as an environment activist called Mark Stone until he was "outed" in January this year, will this week reveal how he has lost everything.

He is no longer with the female protester he came to love, nor with his wife and two children, and he is unemployed: "I don't have any future of a job or any prospect of a job... how can I expect people to ever trust me again?" he says.

Mr Kennedy went undercover in 2003 – spending several years establishing a reputation as a committed activist. He was nicknamed "flash" for his ability to produce money and help with transport to protests. Speaking in a documentary, The Undercover Cop, to be broadcast on Channel 4 tomorrow, Mr Kennedy said: "I was meeting interesting people, doing interesting things... I liked the life that Mark Stone had. I also enjoyed the job that Mark Kennedy had."

What began as a job became something more disturbing when he fell in love with a protester called Megan [not her real name]. "We just really connected... I shared my love and my passion and care for her."

But things were not easy. "I couldn't tell her the truth about my family, where I went to school, all those things you share with a partner.

"She fell in love with somebody, I think, who was lying about who he really was, but the love and the care and the affection that I showed for her really existed, definitely. There was no lie about that at all. She meant, and still does mean, a huge amount to me."

The strain of leading a double life took its toll at home. "I couldn't just switch off Mark Stone and crack on to do all the things that Mark Kennedy was expected to do. You'd want to go home and you'd want to give everything that you possibly had to give to your children, and be there as a father and as a dad – you know, do all the things which you were expected to do – and yet I was struggling with that."

And he recalls how he was beaten up by fellow police officers at a protest at Drax power station, in Yorkshire, in 2008: "I got dragged to the ground and punched and kicked and was stamped on... I felt embarrassed to be a police officer."

Allegations that Mr Kennedy was an "agent provocateur" led to the collapse of a trial involving eco-activists accused of trying to shut down the Ratcliffe-on-Soar coal-fired power station, near Nottingham, in 2009. It was a difficult time. "I had huge anxiety. I wasn't eating or sleeping, and it was really stressful... I felt right out on my own, right out on a limb."

When his subterfuge was exposed, he faced his fellow protesters: "It was definitely an interrogation. I think they felt completely and utterly betrayed... And about half past five in the morning I got up and I walked out of there... and that was it. I haven't seen them again."

He has also left the police: "I was basically told you have no skills and we don't have a suitable job for you... in the end, I thought I can't do this any more, so I resigned."

Mr Kennedy is plagued by guilt. "You're not infiltrating people who are selling crack cocaine or selling a gun or selling children for the sex trade; you're infiltrating people who have a social conscience."